The action of serotonin in the synapse is determined by reuptake that is mediated by a specific, high-affinity, energy-dependant, protease- sensitive transporter located on the plasma membrane of serotonergic cell. Experiments were carried out to characterize mouse mRNA encoding the serotonin transporter of brain, using Xenopus laevis oocyte expression system. It was shown that mouse brainstem is a suitable source of serotonin transporter RNA, and that this tissue contains RNA in the size range of 4.0 - 4.5 kb that can direct uptake of serotonin.